Rings around the Earth: A clue to climate change?
Rings around the Earth?
While most of us know about rings around Saturn and Jupiter, some scientists believe there have been rings of rock debris around our own planet. Two scientists — Peter Fawcett of the University of New Mexico and Sandia’s Mark Boslough (9212) — have suggested that a geologically "recent" collision (about 35 million years ago) may have caused such a temporary debris ring.
The two also suggest that such temporary rings — lasting from 100,000 to a few millions of years — may explain some patterns of climate change observed in the Earth’s geological record. These conclusions are spelled out in an upcoming article in the Journal of Geophysical Research, Atmospheres.
Lore of the Rings
"One way to get a ring," says Mark, "is with an impact." There is a growing body of evidence showing that the Earth has been subjected to numerous impacts by comets and asteroids throughout its history. Among these impacts are Meteor Crater in Arizona, the buried Chixulub crater in the Yucatan Peninsula of Mexico, and a chain of at least five craters spread across several continents.
Several studies, both theoretical and with laboratory data, suggest that some large impacts are capable of ejecting material into space in the form of debris rings, if the mechanics of the impact meet certain requirements. The authors conclude that the mostly likely scenario for ring creation is a low-angle impact by a fiery meteor. Some Earth materials and melted meteoric debris, called "tektites," would form the ring materials.
Mark and Peter describe an impact where the meteor ricochets back into the atmosphere. The ricochet becomes part of an expanding vapor cloud, setting up an interaction that allows some of the debris to attain orbit velocity. The orbiting debris will collapse into a single plane by the same mechanics that led to the rings of Saturn and other planets, Mark explains. Such a ring would most likely form near the equator, because of the dynamics involved with the moon and the Earth’s equatorial bulge.
Speculation on climates past
The effects of the larger impact events on Earth’s environment and climate have been the subjects of much speculation and research over the past two decades. "Clearly, large impacts have affected the evolution of the Earth, life on it, and its atmospheric environment," says Peter, who teaches paleoclimatology, mathematical modeling, and environmental science in UNM’s Earth and Planetary Sciences Department.
Much of the paleoclimate work has focused on the Cretaceous-Tertiary (K-T) boundary event, which marked a mass extinction and the end of the age of the dinosaurs. A number of these studies suggest an impact resulting in the suspension of a layer of dust in the upper atmosphere, blocking sunlight and cooling the Earth. But could other impacts result in different atmosphere-altering phenomena?
An equatorial ring would cast a shadow primarily in the tropics, as it does for Saturn. (See illustration on page 4.) Depending on location, surface area, and darkness of the ring shadow, the amount of incoming solar warmth, or insolation, could be significantly altered, the two concluded. To test their theory, the two assumed an opaque ring, like Saturn’s B-ring, scaled to Earth-size, and tested global climate effects using a climate model.
The model selected and modified for the simulation was developed by the National Center for Atmospheric Research (NCAR). NCAR’s "Genesis" climate model includes atmospheric circulation information and layers of vegetation, soil, snow, sea temperature, and land ice data.
"The idea for this project was to write a distributed-memory parallel version of this existing and popular climate code, so we could run it on machines with Cplant-type architectures," Mark explains. "From Sandia’s point of view, we wanted to gain experience with the code and develop collaborations with the paleoclimatology research community."
Sandia funded Mark’s work on the program through its Laboratory Directed Research and Development (LDRD) program and Peter’s efforts through the Sandia University Research Program (SURP). Mark accomplished the rewriting of the code and crunched the ring data. Peter shared his expertise to analyze the results.
"The equatorial debris ring has a profound effect on climate, because it reflects a significant fraction of tropical insolation back to space before it can interact with the atmosphere," the pair conclude. Surface and atmospheric temperatures, changes in temperature ranges from equator to poles, circulation patterns, and the rain and snow cycles were all impacted by the ring, their model showed.
The two scientists looked at changes shown in the model to predict changes that might be found in the Earth’s geologic record as a way to test their work. In addition to the K-T boundary event, they looked at more recent impacts and a much older one.
The most recent event — about 35 million years ago — is identified by an iridium layer (often associated with meteors) and two pronounced micro-tektite fields, where these melted meteor-related materials have been found and dated. Climatic records from sedimentary materials just above the iridium/micro-tektite interval indicate a 100,000-year cooling interval. Orbiting debris in a ring, casting its shadow in the subtropics, could have sustained such a cooling trend, the two authors suggest.
The K-T boundary impact (about 65 million years ago) was much larger than the more recent impact and had a much larger immediate effect on the environment as measured by extinctions and atmospheric changes. But there were no long-term effects on the climate, leading the authors to conclude the event probably did not generate a debris ring.
This large-impact without a ring underscores the importance of the geometry of an Earth collision, Mark explains. When the size of the impacting body is below some critical dimension, the impact must be at a shallow angle to create ring debris.